1. Field of the Invention
This invention relates generally an oxidation/reduction catalyst. It relates particularly to an improved oxidation/reduction catalyst for the treatment and control of post combustion emissions.
2. Description of the Related Art
Emissions from fossil-fuel combustion contribute significantly to smog, acid rain, and global warming problems and are subject to stringent environmental regulations. The most significant regulated emissions include CO, CH4 and unburned hydrocarbons (HCs), and NOx. These regulations are expected to become more stringent as state and regional authorities become more involved in addressing these environmental problems. Better systems are needed for catalytic control. Exhaust emission composition characteristics of fossil-fuel burning internal combustion engines vary with air-fuel mixtures. The most energy-efficient operation is under stoichiometric conditions where exhaust oxygen levels are less than 1.0 vol %. Under this condition, exhaust gas temperature is much higher and HCs and NOx levels are much higher than for operations under lean-burn conditions. In order to better control emissions, modern engines are operated under lean-burn conditions to minimize CO and NOx emission levels so that catalytic converters are better able to reduce them below regulatory levels. In all air-fuel mixtures, CO and HC levels are considerably higher than NOx levels (Ref. Gas Research Institute RFP #94-260-0470, 1994) and are potential candidate reagents for the reduction of NOx to N2 with a catalyst which catalyzes the reducing chemical reduction.
In general, existing catalytic converters used for NOx and HC emission control use precious metal or combinations (PM's) as wash coats with various architectures over alumina on ceramic substrates to effect catalytic conversion. Some of the more common are coatings of Pd, Pd/Rh, or Pt/Rh.
Existing catalytic converters are less effective for removal of methane HC emissions due to the high light-off temperatures for methane on these catalysts. The greater challenge to existing emission control technology is ultra-low HC and NOx emission performance with higher converter operating temperatures (e.g. near stoichiometric air-fuel mixtures for more efficient engine operation) (Ref. Manufacturers of Emission Controls Association: Advanced Emission Control Technologies for LEV 2 Standards Meeting, May 1998).
In response to the need for the next generation of catalysts for automotive applications, low-temperature oxidation catalysts were developed by NASA Langley Research Center. These improved catalysts are described in U.S. Pat. Nos. 4,829,035; 4,839,330; 4,855,274; 4,912,082, 4,991,181, 5,585,083; 5,948,965 and 6,132,694 and are hereby incorporated by reference as if set forth in their entirety herein. These catalysts exhibit several key advantages over the current state-of-the-art. First, unlike the thick, inert layer of alumina used in conventional catalyst technology, these catalysts can use a single active tin-oxide coating (<5 microns) that enhances the catalytic performance by acting as an oxygen storage device. Second, their active washcoat reduces the temperature (i.e., light-off) at which the catalyst begins converting pollutants (e.g. CO, HC and NOX) to non-pollutant gases, as well as, requiring less precious metal to attain the same pollutant gas conversion efficiency over time. Third, these catalysts are capable of capturing enough oxygen from the natural exhaust stream to complete the chemical reactions. Unlike traditional catalytic converter technology, external air sources and the ancillary sensors, air pumps, and hoses are not required for catalytic converter operation.
Despite these advances in catalyst technology, there remains a need for an improved catalyst capable of oxidation and reduction for a variety of applications. Specifically, there remains a need for an oxidation/reduction catalyst for use in diesel and natural gas applications as well as non-automotive pollution sources.